[PATCH] D105723: [LSR] Do not hoist IV if it is not post increment case. PR43678

[Serguei Katkov via Phabricator via llvm-commits]

skatkov added a comment.

Some additional information to help to reviewers, in other words why I think this is a good fix.

LSR, in LSRInstance::OptimizeLoopTermCond(), sets the IVIncInsertPos variable to common dominator of all conditions related to post-increment IV.
Say, if we have a loop exit with IV check which is not latch and increment of this IV is located in latch. The optimization will try to hoist IV increment closer to check to increase the chances to utilize the same physical register for increment and check.

Later, for each IV it adjust insert position in LSRInstance::AdjustInsertPositionForExpand and in this place it insertion position is adjusted taking into account whether IV is post-increment or not. So for post increment all pieces of formula will be expanded before IVIncInsertPos.

However if IV is not post incremented insertion position will not be adjusted.

Later in SCEV expander, there is another attempt to hoist IV increment (this part which is in the patch) and it does not take into account whether current IV is post-increment or not.
it just based on IVIncInsertPos variable which is set once for all IVs processed in LSR.

As a result if current IV processed by SEV expander is not post incremented (but there is another IV with post-increment), IV increment expansion will be hoisted while other parts of formula do not. This causes a bug.


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https://reviews.llvm.org/D105723